MITL control of remote aerial vehicles provides a level of accuracy and real-time adaptability which fully autonomous systems generally cannot provide. Thus, MITL control is commonly used to designate a desired target for an airborne missile. MITL control of an airborne missile is typically accomplished by a terminal sensor mounted in the nose of the missile which transmits sequential video images of the target area, via the video portion of a communications data link, to a control center.
The control center is equipped to receive and display the sequential video images to an operator. The operator designates the target, or aimpoint, from the video images on the display. The position of the target on the display is then transmitted, in the form of a target designation signal, to the missile via the command portion of the data link.
In some cases, the operator is required to actually fly the missile to the designated target by continuously attempting to center the target on the video display. While in other cases, once the target is designated, the terminal sensor on the missile provides the necessary steering signals to guide the missile to the target. In the case in which the terminal sensor guides the missile, a guidance tracker typically provides a measure of the relative angular velocity between the missile and the target. The relative angular velocity is then used by the on-board guidance and navigation computer to fly the missile to the target.
Although MITL control has been used to successfully designate targets in various missile systems, conventional MITL control methods suffer from a number of deficiencies. Individually, and as a whole, these deficiencies reduce the overall effectiveness and user-friendliness of the missile system.
Most importantly, the inherent time delays in the data link between the missile and the control center may cause the MITL control loop to become unstable. In a conventional MITL system, the operator typically issues angular rate commands to the missile to position the terminal sensor over the target. Because of the time delay between designation of the target and the response of the terminal sensor, the operator generally issues azimuth and elevation gimbal rate commands which are greater than necessary to point the terminal sensor toward the target. When this happens, the sensor may overshoot the target, resulting in a pointing error. The operator is then likely to once again input gimbal rate commands which are greater than necessary to correct the overshoot and return the sensor to the proper pointing position. Each time the operator attempts to correct the cumulative overshoot, the gimbal rates diverge farther from the rates necessary to point the terminal sensor toward the designated target. As a result, the control loop becomes unstable. Even if the inherent time delays do not cause the control loop to become completely unstable, their effect is to destabilize the target designation process, and thus, they reduce the overall effectiveness of the MITL missile system.
For missiles which include a terminal sensor having a guidance tracker for providing steering signals to guide the missile to the desired target, the target which is tracked may not be the same as the target designated by the operator. For example, some trackers, known in the art as centroid trackers, provide steering signals to guide the missile to the centroid of a defined area in the image of the target. Typically, the geometry of the defined area is determined by thermal contrast of the target area as measured by infra-red sensors. The operator, however, may have selected a particular feature, other than the centroid, as the target. Although other guidance trackers define the image of the target differently, they often share the same characteristic error. As a result, they too can guide the missile to a target other than the designated target. In many cases, designating a desired target within the defined area of the target image is further complicated by characteristics of the target itself, the weather, and other uncontrollable variables.
Another problem inherent with conventional systems for MITL control is that the video images provided to the operator are generally not stable. Individual images are typically transmitted from the missile to the control center at a rate of from about ½ to about 30 frames per second. At the same time, the terminal sensor is continually subjected to the destabilizing forces generated by the flight of the missile. Accordingly, even at faster transmission rates, the images displayed to the operator can appear to flicker on the display. The flickering images on the display increase the skill level of the operator necessary to effectively designate the desired target.